1. Field of the Invention
This invention relates to polyurethane prepolymers of an isocyanate component and a polyol component. These isocyanate-terminated prepolymers are constituents of formulations for moisture-hardening foams.
2. Description of Related Art
Polyurethane prepolymers have long been known. They are used in various compositions in the adhesives field. Recently, however, they have been used to an increasing extent for formulations for the production of foams which have acquired increasing significance in the construction field by virtue of their moisture-, cold- and sound-insulating properties. So-called "one-component" polyurethane foams are preferred for use in such applications. The one-component polyurethane foams are mixtures of polyurethane prepolymers, accelerators, viscosity reducing agents, blowing agents and other auxiliaries which are marketed in moisture proof pressurized containers and which harden on release from these pressurized containers, i.e. on contact with atmospheric moisture or water vapor, to form solid foams.
U.S. Pat. No. 3,830,760 proposes a process for the production of polyurethane foams of this type. To this end, a polyurethane prepolymer is prepared from tolylene diisocyanate or crude diphenylmethane diisocyanate and a polyol having a molecular weight of at least 300. On release from a suitable pressurized container, this prepolymer expands and then hardens in the presence of the moisture in the surrounding atmosphere. The polyol used is a polyol formed from an alkylene oxide by reaction with compounds containing several active hydrogen atoms. Prepolymers prepared using other suitable polyol components are also described.
According to U.S. Pat. No. 4,258,140, polyurethane prepolymers intended for the same purpose may also be prepared using polyester and/or polyether polyols containing tertiary amino groups and from 2 to 8 hydroxy groups as the polyol component.
To produce the foams, the prepolymer mixtures mentioned in the publications cited above are foamed with fluorinated and/or chlorinated hydrocarbons which have boiling points at atmospheric pressure preferably below room temperature.
All the hitherto mentioned foams produced by the disclosed processes show unsatisfactory fire behavior. In the fire test according to DIN 4102, Part 1, they have to be classified as "readily inflammable" (classification B3). The typical maximum flame heights of foams such as these are from 20 to 30 cm. Accordingly, building regulations often stipulate that foams of the type in question should not be used in building construction for safety reasons. Accordingly, there is a need for polyurethane foams having improved fire behavior (DIN 4102 classification: at least B2 "normally inflammable"). Although a certain improvement in fire behavior may be obtained with bromide-containing flameproofing agents, this would involve other disadvantages, including an adverse effect on the hardening reaction and instability of the prepolymers through an increase in viscosity in the mixture.
U.S. Pat. No. 4,508,853 describes polyurethane prepolymers which are free from most of the disadvantages mentioned above. The polyol components of the prepolymer mixtures disclosed are oleochemical polyols which have been obtained by complete or partial ring-opening of epoxidized triglycerides (fats or oils of native origin) with monohydric or polyhydric alcohols. These oleochemicals polyols which originally contained some olefinic unsaturation for epoxidation optionally contain residual epoxide groups and have an overall functionality (OH and epoxide groups) of from 2.0 to 4.0. Accordingly, the polyol components disclosed in this publication are derivatives of triglycerides of native origin which originally contained olefinic double bonds, are epoxidized by known methods and have to be completely or partly reacted with monohydric or polyhydric alcohols. The fire behavior of the polyurethane foams produced from polyurethane prepolymer mixtures containing polyols such as these tends to be more favorable than that of the compositions of U.S. Pat. Nos. 3,380,760 and 4,258,140 mentioned above. However, the mixtures show a marked increase in viscosity with decreasing temperature, and since processibility at low ambient temperatures is important and critical for use in the construction field, further improvement in viscosity behavior is desirable.